Dielectric microresonators have attracted increasing attention in opto-electronic and sensing applications, including biosensing. One common configuration of microresonator involves a glass microsphere, typically 20 μm to a few millimeters in diameter, which is put into close proximity to an optical waveguide such as an optical fiber that has been heated and tapered, or etched to a total thickness of 1-5 μm.
The tapering modifications to the fiber result in there being a substantial optical field intensity outside the fiber, and thus light can couple into the microsphere and excite its eigenmodes, often referred to as whispering gallery modes (WGMs). When microresonators are made with low loss materials and have a high surface quality, the propagation loss of light propagating in WGMs may be very low, and extremely high quality factors, also known as Q-factors, can be achieved: values as high as 109 are achievable. Due to the high Q-factor, the light can circulate inside the microresonator for a long time, thus leading to a large field enhancement in the cavity mode, and a long effective light path. This makes such devices useful for linear, non-linear and optical sensing applications.
There are practical difficulties in realizing the fiber-microsphere combination described above. First, the fiber must be tapered to a few microns in diameter. This commonly results in a relatively long (a few cm) and fragile tapered region. Second, the relative position of the microsphere and the fiber taper must be held constant to within a few nanometers if the optical coupling and the Q-factor are to remain constant. This is difficult with a free sphere and thinned fiber.
Other forms of micro-optical resonators have used a disk, or ring, rather than a sphere as the optical resonant cavity, where the disk and waveguide are fabricated on the same planar substrate. This monolithic approach is typically realized in semiconductor waveguides, and provides excellent stability of coupling between the waveguides and the resonator. The etching process used to fabricate the disk resonator, however, invariably introduces surface roughness, that results in a scattering loss that severely degrades the Q of the cavity. Cavities formed using this approach typically have a Q-factor value of around a few thousand.
Another approach is to suspend a glass microsphere above the surface of a channel waveguide fabricated on a planar substrate, so that the optical coupling between the sphere and the waveguide takes place in the vertical direction. This approach preserves the high Q-factor of the glass microsphere, but does not solve the problem of how to precisely control the coupling between the microsphere and the waveguide.